"No animal has ever evolved anything quite like this," said lead scientist Dr Nicholas Longrich, from Yale University in Connecticut, US.

"We don't know of any other species that uses its body like a flail. It's the most specialised weaponry of any bird I've ever seen."

Xenicibis was a relative of the ibis that lived 10,000 years ago and is only known to have inhabited Jamaica.

The size of a large chicken, it was similar to other members of the ibis family apart from its wings.

They contained hand bones that had evolved into powerful club-like weapons, scientists discovered.

"When I first saw it, I assumed it was some sort of deformity," said Dr Longrich, who led a study of recently discovered partial Xenicibis skeletons. "No-one could believe it was actually that bizarre."

Xenicibis also had a much larger breastbone and longer wings than most flightless birds.

"That was our first clue that the wings were still being used for something," said Dr Longrich.

The research is published today in the journal Proceedings of the Royal Society B.

Other birds are known to defend themselves with their wings, but only Xenicibis used its hands - hinged at the wrist joint - as swinging flails.

Evidence suggests that, like modern ibises, the birds were highly territorial and fought over nesting and feeding rights.

Two wing bones in the collection examined by the scientists bore signs of combat. One hand bone was fractured, and a centimetre-thick upper arm bone was broken in half.

The injuries were proof of the extreme force the birds were able to generate with their specialised wings, said Dr Longrich.

The weapons may also have been used to defend eggs and young against predators, the scientists believe.

Xenicibis was threatened by a number of predators, including the Jamaican yellow boa, a small extinct monkey and more than a dozen birds of prey.